Method for laminating glass sheets using microwave radiation

ABSTRACT

A method for laminating glass sheets wherein laminating film is placed over one surface of a first glass sheet and the film is heated with microwave radiation to a bonding temperature. Heated areas of the film are successively pressed to the glass sheet in a continuous manner for purging air from between the film and the first glass sheet and for applying bonding pressure. The pressed film areas are then cooled whereby an appropriate bond is attained between the film and the first glass sheet. Thereafter the first glass sheet with its bonded film is subjected to a partial vacuum and a second glass sheet is positioned on the film and the second glass sheet is pressed to the film. The film is then reheated with microwave radiation to a bonding temperature and thereafter cooled whereby an appropriate bond is obtained between the film and the second glass sheet to provide a glass lamination.

CROSS REFERENCE

This application is based upon Provisional Application No. 60/536,338,filed on Jan. 13, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for laminating glass sheetsand other frangible materials, wherein a plastic film is sandwichedbetween sheets.

Flat or non-flat sheets of glass, ceramics, polymers, or combinations ofthese materials may be laminated in accordance with the teachings of thepresent invention.

2. Discussion of the Prior Art

Laminates provide a way of strengthening frangible material, for exampleglass, so as to extend its uses and to render it safer to use in certaincircumstances. Thus laminated glass products can be used for automotiveand aircraft glazing, glass doors, balustrades, bulletproofing and manyother uses where the glass product must be strong and/or shatterproof.

In conventional laminated glass products a sheet of glass is bonded to alayer of polymer adhesive film, and a further sheet or layer of materialis bonded to the other side of the adhesive film layer, so that theadhesive film is sandwiched between two outer layers. If the glass sheetis then struck a blow it cracks or breaks, but does not shatter intosmall hazardous sharp pieces as the broken pieces are still bonded toand held in place by the polymer layer. If the laminated glass is usedin a car windscreen, therefore, occupants of the car are not showeredwith broken glass upon breakage of the windscreen.

A number of methods for producing such laminates have been disclosed.For Example, see US Pat. Nos: 5,268,049; 5,118,371; 4,724,023;4,234,533; and 4,125,669. Laminated glass has been generallymanufactured by a process wherein a stack of at least two sheets ofglass having a plastic film called an intermediate film or laminatingfilm, typically a plasticized polyvinyl butylal (PVB) film, issandwiched between each pair of adjacent sheets of glass which issubjected to evacuation, pressing and heating.

Usually this involves long heating under temperatures of around 80°C.-140°0 C. and high pressure, 4 MPa-20 MPa. The main problem encounteris that air is trapped between the film and glass surfaces, which airmust be removed. This is required to prevent the laminate from bubbling.Removing the remainder of the air requires long heating and highpressure. The bubbling is a visible and objectionable defect that inmost cases is absolutely unacceptable. Besides, bubbling within thelaminate may reduce its strength in this area and cause delamination.

At the same time removing air is not an easy task because it is trappedbetween both sides of the plastic film and glass sheet and there areonly two mechanisms by which the air can escape: diffusion anddissolving in the film. Both processes are very slow, requiring longterm heating and applying of high pressure. The bigger the glass sheet,the longer the time required. An especially long time is required formaking multi-layer laminates. As a result, the productivity of suchprocesses is low and they require considerable capital expenditure toset up the necessary costly apparatus such as autoclaves.

Many prior art patents focus on the solution of problems related to theair escaping. In U.S. Pat. No. 5,268,049, glass sheets are spaced apart,and in the method described by U.S. Pat. No. 5,268,049, a liquid resinis used. In U.S. Pat. No. 4,234,533 the two sheets are held at an angleand in U.S. Pat. No. 5,118,371 the thickness of PVB gradually increases(or decreases) from the one side to the other side of the glass sheets.In U.S. Pat. No. 3,509,015 a method is described for producing laminatedglass by sealing the periphery of two parallel glass sheets withpressure sensitive tape and forcing resinous material under pressureinto the inter-sheet space. The resinous material is forced through aself-closing valve held in place with the tape while trapped air escapesthrough an aperture in the taped seam at the top of the cell. U.S. Pat.No. 4,125,669 describes a similar method in which two glass panes aresealed all around except for a filling opening and an aeration opening,and a binder material is introduced into the envelope thus formed in anamount calculated to exactly fill the envelope. Putty is applied to theopenings just before emergence of the binder upon laying the filledenvelope flat.

U.S. Pat. No. 3,315,035 describes a method involving the maintaining ofthe glass sheets in opposite relationship, heating the sheets to about200° F. and injecting a resin composition containing a hardening agent,preheated to about 200° F., into the inter-sheet space and curing theassembled article. In U.S. Pat. No. 4,234,533 the seal around the sheetsis formed by a gas-permeable, resin-impermeable material such as“Scotchmount”. In some inventions (see for example U.S. Pat. Nos.4,828,598 and 4,724,023) the laminating process is conducted in avacuum. The vacuum environment helps air to escape and, in general, canreduce the level of trapped air. However, heating in a vacuum is alwaysdifficult, inefficient and therefore the laminating process stillrequires a long time.

Thus, all the above described methods of air bubble removal, are notfully effective and still require long term heating (high energyconsumption) and special expensive equipment, such as high pressureautoclaves.

At the same time, extremely large numbers of windshields, windows andother laminate products are made each year. Accordingly, there is aclear need in the art for a more effective and less expensive method forlaminating glass sheets which eliminates expensive equipment and reducesenergy consumption.

SUMMARY OF THE INVENTION

According to the present invention, a method is provided for laminatingglass sheets and other frangible material with the thermal treatment ofa laminating film that is processible by controlled heating which isfast and does not require the use of autoclave type furnaces. Productsprepared using the method of the present invention include, but are notlimited to, architectural glass, glass doors, balustrades, bulletproofglass, windshields, side windows and rear windows for vehicles such asautomobiles and the like, as well as many other uses where the glassproduct must be strong and/or shatterproof and comparable products. Theinventive method utilizes microwave radiation to rapidly apply heat tothe adhesive film to be thermally treated.

In the method for laminating glass sheets in accordance with theteachings of the present invention, laminating film is placed over onesurface of a first glass sheet and the film is heated in a continuousmanner with microwave radiation to a bonding temperature. Thereafterareas of the heated film are successively pressed to the first glasssheet in a continuous manner for purging air from between the film andthe glass sheet, and for applying bonding pressure. The pressed filmareas are then cooled whereby an appropriate bond is attained betweenthe film and the glass sheet.

The first glass sheet with the applied bonded film is then subjected toa partial vacuum and a second glass sheet is positioned on the film.During this process the second glass sheet is pressed to the film andthe film is reheated with microwave radiation to a bonding temperature.Thereafter the reheated film is cooled whereby an appropriate bond isattained between the film and the second glass sheet thereby providing aglass sheet lamination.

The laminating process of the present invention may also be carried outwith a stack of glass sheets. In this embodiment multiple of first glasssheets with applied bonded film are stacked in the partial vacuumwhereby non-coated surfaces of the first glass sheets engage the filmbonded to an adjacent first glass sheet, leaving one bonded film leftexposed. The second glass sheet is positioned on the exposed film of thestack and the steps of reheating the film and cooling the reheated filmare each carried out on all film layers in the stack simultaneously.

In the initial step of placing the laminating film over one surface ofthe first glass sheet, in accordance with the teachings of the presentinvention, an edge of the film may be fixed to a corresponding edge ofthe first glass sheet and then the step of heating is initiated at thefixed edge and continuously advance therefrom over the film. In carryingout this procedure, it is desired to have an initial gap between thefilm and the first sheet and the sheet is then successively andprogressively pressed to purge the air with a non-stick applicator. Thesteps of heating and pressing the film to the first glass sheet may beprovided successively in a continuous manner by moving a combinationmicrowave heater-roller over the film.

The microwave radiation frequency is generally selected to be betweenapproximately 0.9 GHz to approximately 200 GHz. The microwave radiationwavelength is preferably selected to be between approximately fouroptical thicknesses of the first glass sheet for the selected wavelengthto approximately the sum of the thicknesses of skin layers in the filmand the first glass sheet.

The initial heating of the film to the first glass sheet may also becarried out in a partial vacuum and the vacuum level in this stage or inthe reheating stage is selected whereby remaining air in the laminatedoes not create visible defects. Additional electromagnetic heat mayalso be applied by an additional source with a wavelength that issignificantly shorter than the applied microwave radiation used toreheat the film. To enhance the process of heating the film, a metalreflector may be positioned behind the first glass sheet.

The method of the present invention avoids the use of expensive andinefficient autoclave type furnaces and allows conducting the laminatingprocess continuously with high production rate and low energyconsumption.

The main advantages of this high speed method are reduction ofmanufacturing costs and increase of production rate. Other advantagesexist, such as production yield and providing an opportunity for processautomation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages appear hereinafter in the followingdescription and claims. The accompanying drawings show, for the purposeof exemplification, without limiting the invention or appended claims,certain practical embodiments of the present invention wherein:

FIG. 1 is a schematic drawing illustrating heating of the film bymicrowave with progressive pressing and cooling in a continuous mannerin accordance with the teachings of the present invention;

FIG. 2 is a schematic drawing illustrating progressive and simultaneousheating and pressing and subsequent cooling of the film by microwavethrough a roller in a continuous manner in accordance with the teachingsof the present invention;

FIG. 3 is a schematic drawing illustrating progressive and simultaneousheating and pressing and subsequent cooling of the film wherein themicrowave heating is provided through a roller with a heat distributionarea in the shape of a strip in accordance with the teachings of thepresent invention;

FIG. 4 is a schematic drawing illustrating the reheating and pressing ofa stacked laminate in accordance with the teachings of the presentinvention; and

FIGS. 5 a and 5 b are schematic drawings and corresponding graphsillustrating the microwave power distributions respectively inside ofthe first glass sheet and film combinations of FIGS. 1 and 2 and insideof the laminate package of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method of laminating frangiblematerials, preferably glass sheets, without using autoclave typefurnaces to effect rapid and exclusive microwave heating of the film. Inthe invention the laminating film (1) (see FIG. 1) is placed over thefirst glass sheet (2) with a gap (6) and is fixed it to one edge (3) ofthe glass sheet. Selected areas of the film, which can be the entirefilm or a local portion of it, are exposed to microwave radiation (4)which heats it to a sufficient bonding temperature. The heated filmareas are successively non-stick pressed to the first glass (2) by wayof a moving pressing zone (5) in a continuous manner. Pressing beginsfrom the area where the film is fixed to edge (3) in a direction towardthe opposite edge. The ability to successively and continuously pressthe rapidly heated film area in combination with the ability to maintaina gap (6) between glass and film in front of the pressing zonesignificantly facilitates and accelerates the air removal process fromgap (6). Cooling of the film (1) follows pressing and can beaccomplished in many typical ways such as by cold air stream (7) behindthe pressing tool, by the pressing tool itself or by a portion of it,etc. As a result an appropriate bond between the entire film and thesurface of the first glass sheet can be obtained.

Speed and quality of bonding increases if heating and pressing areprovided simultaneously. For this the microwave radiation (4) (FIG. 2)heats a local area of the film (1) through the pressing tool (9) that ismade from materials that are transparent to the microwaves. Among suchmaterials are Teflon, quartz, oxide ceramic, or the like. The easiestway of simultaneously heating and pressing is to provide a local heatarea in the shape of a strip (8) (see FIG. 3) and pressing is providedby a tool (9) in the form of a roller made from Teflon, quartz, oxideceramic, or the like.

When the film (1) (see FIG. 4) is bonded to the first glass sheet (2) itis placed in a partial vacuum, which is illustrated by the surroundingspace of the drawing, and a second glass sheet (10) is placed above (onthe top of) the film (1) and the second glass sheet (10) is pressed tothe film (1) as indicated at (11) while the film (1) is reheated bymicrowave (12) to sufficient bonding temperature. Then recooling (whichis not illustrated), is performed to create the laminate. The laminatingprocess is rapid because the vacuum level is selected so that theremaining air does not create visible defects in laminate and there isno requirement for air removal. There is no need to apply high pressureand long heating for dissolving air, as in the current technologies.Practically, the vacuum level should be no greater than one kPa. Thisdoes not require expensive pumps and can be achieved in seconds even forlarge chambers of many cubic meters.

Speed of laminating increases if reheating and repressing are providedsimultaneously. To accomplish this the microwave radiation heats thefilm through the pressing tool itself which is made from materials thatare transparent to the microwaves. Among such materials are Teflon,quartz, oxide ceramic, or the like. The easiest way of simultaneouslyreheating and pressing the second glass sheet is to provide the localheating area in the shape of a strip.

In the embodiments of the invention discussed above, reheating time aswell as cost can be reduced by at least 10% to 20% by using microwaveradiation and an additional electromagnetic heat source, for example aninfrared source.

In the embodiments of the invention discussed above, production rateincreases and capital costs decrease by placing the first glass sheetwith the film placed over it in a vacuum before heating. In this casethe glass moves only once into position (to the vacuum chamber) wherethe heating and reheating processes are provided. The vacuum chamber isused as a microwave chamber.

In the method of the present invention, microwave radiation withappropriate frequency (wavelength) is used. In all of the embodiments ofthe present invention, the wavelength of the applied microwave radiationis an important parameter that must be determined for each type andthicknesses, both of the film and glass sheet being processed. Theparticular frequency chosen should ensure maximum selectivity of directheating of the film through the thickness of the second glass sheet. Inaddition, the chosen frequency should be cost effective and microwavegenerators for the selected frequency should be readily available at therequired power.

When microwave radiation is applied to a film placed over the glass orto the first and the second glass sheets with the film between, themicrowave radiation passes through the film and glass sheets and heatsall of them. The portion of the energy that is absorbed by the film andby the glass sheets depends on the microwave frequency, absorptionproperties of the film and glass sheets and their thicknesses. The filmand glass absorption properties are usually close for microwavewavelengths larger than the skin layer in the film and glass for thesewavelengths. In this case, the processed areas of the film and glasssheets are heated to approximately the same temperature and the heatingtime depends on power density. The more power density, the faster thefilm can be bonded to the first glass sheet. However the necessary powerdensity drastically rises if the microwave frequency is lower than 0.9GHz, and this creates many technical and economic problems.

Using microwave with shorter wavelengths (higher frequency) reducesheating and reheating time and increases efficiency of the processes.Total microwave energy is coupled inside the film and glass in thiscase. However microwave generators on a frequency higher than 200 GHzfor the necessary power are not available.

Therefore, the microwave frequency range for the present invention isgenerally between about 0.9 GHz and about 200 GHz. Selecting amillimetric wavelength range for the microwave radiation allowsgenerators that produce concentrated controllable power (such asgyrotrons) to be used. Using this wavelength range also allows thefocusing of the microwave power to heat the local area of the film.

Efficiency and speed of the film heating and reheating increases ifmicrowave radiation wavelength is selected to be about four opticalthicknesses of the first glass sheet for the selected wavelength. Inthis case a standing wave distribution of microwave power is formed(see, for example, Principles of Optics: Electromagnetic Theory OfPropagation, Interference And Diffraction Of Light by Max Born and EmilWolf; with contributions by A. B. Bhatia [et al.]. 7th expanded ed. NewYork: Cambridge University Press, 1999.). FIG. 5 a illustrates energydistribution E in a glass sheet along the depth z during the heating offilm (1) and in FIG. 5 b the same distribution illustrated for thereheating process. These wavelengths are most preferred for heating andreheating film because they provide the most effective way to heat film(the required power level drops by several times); i.e., a comparativelyshort heating time can be achieved using a reasonable microwave power.

The standing wave type of microwave power distribution is a result ofinterference between the transmitted microwaves and those reflected fromthe opposing external surface of the glass sheet. In one embodiment ofthe invention, a reflector is placed behind the first glass sheet at adistance equal to 0, 1, 2, 3 . . . multiplied by half wavelengths in avacuum corresponding to the selected frequency. The reflectorintensifies this interference. A special metal plate, fixture or aportion thereof, and the like used for supporting the first glass sheetcan be used as a reflector.

Efficiency and speed of the film heating is further increased ifmicrowave radiation wavelength is selected to be about the sum of theskin layers in the film and the first glass sheet. In this case only thefilm and contiguous (bordering) layer of the first glass sheet areheated but not the entire glass in depth.

The present invention includes laminating more than two glass sheets aswell. In this case the first, second, third and so forth glass sheets,except the last one, are covered separately by the film with a gap andeach film is fixed to one edge of the correspondent glass sheet. Theneach film is separately exposed to microwave radiation to heat selectedfilm areas to sufficient bonding temperature, successively in acontinuous manner, by non-stick pressing of the heated area of each filmto the glass by a moving pressing zone, followed by cooling the pressedfilm areas of each film. As a result air removal from each selectedarea, as well as, appropriate bonding between the entire film and thesurface of the corresponding glass are obtained. The microwave frequencyrange for the present invention is generally between about 0.9 GHz andabout 200 GHz.

Speed and quality of bonding also increases in this embodiment ifheating and pressing of each film is provided simultaneously.Accordingly, the microwave radiation heats selected film areas of thefilm through the pressing tool (as is shown in FIG. 2) which is madefrom materials that are transparent to the microwaves. Among suchmaterials are Teflon, quartz, oxide ceramic, or the like. The easiestway of simultaneously heating and pressing each film is to provide thelocal heat area in the shape of a strip (as is shown in FIG. 3) and toaccomplish pressing by a roller made from Teflon, quartz, oxide ceramic,or the like.

Efficiency and speed of the film heating also increases in thisembodiment if microwave radiation wavelength is selected to bepreferably about four optical thicknesses of the correspondent glasssheet for the selected wavelength. In this case, a standing wavedistribution of microwave power is formed to the maximum on the film (asshown in FIG. 5). These wavelengths are most preferred for heating filmsbecause they provide the most effective way to heat them.

When the films are bonded to the first, second, third and subsequentglass sheets, they are stacked together and placed in a sufficientbonding vacuum. The first glass sheets with bonded film are successivelyplaced on top a preceding glass sheet with the bonded film thereoffacing toward the next glass sheet of the stack. The second glass sheetis then placed on the top of the film with the non-coated surface of thesecond glass sheet contacting the film to create a package or stack witha selected number of glass sheets and interposed films. The package ispressed and reheated in the partial vacuum by microwave to sufficientbonding temperature and re-cooled to create the laminate. The laminatingprocess is rapid because the vacuum level is selected so that remainingair does not create visible defects in the laminate. Practically, thevacuum level should be one kPa at the most and the frequency rangeshould be between about 0.9 GHz and about 200 GHz. The preferablemicrowave radiation frequency is selected whereby the temperaturevariation of the stacked films does not exceed the permitted level.

In the embodiments of the invention discussed above, reheating time aswell as cost can be reduced by at least 10% to 20% by using microwaveradiation and an additional electromagnetic heat source with awavelength that is significantly shorter than that of the appliedmicrowave, for example infrared.

The present invention has been described in an illustrative manner. Itis to be understood that the terminology that has been used is intendedto be in the nature of words of description rather than of limitation.Many modifications and variations of the present invention are possiblein light of the above teachings. Therefore, within the scope of theappended claims, the present invention may be practiced other than asspecifically described.

1. A method for laminating glass sheets, comprising: placing laminatingfilm over one surface of a first glass sheet; heating said film withmicrowave radiation to a bonding temperature; successively pressingareas of the heated film to said one surface in a continuous manner forpurging air from between said film and said one surface and for applyingbonding pressure; cooling the pressed film areas whereby an appropriatebond is attained between said film and said one surface; subjecting saidfirst glass sheet with bonded film to a partial vacuum and positioning asecond glass sheet on said film and pressing said second glass sheet tosaid film; reheating said film with microwave radiation to a bondingtemperature; and cooling said reheated film whereby an appropriate bondis attained between said film and said second glass sheet.
 2. The methodof claim 1 wherein the step of placing laminating film over one surfaceof a first glass sheet includes fixing an edge of said film to acorresponding edge of said first glass sheet, and the step of heatingsaid film is initiated at said fixed edge and continuously advancedtherefrom over said film.
 3. The method of claim 1 wherein said film isplaced over said one surface with a gap therebetween prior tosuccessively pressing.
 4. The method of claim 1 wherein the step ofsuccessively pressing is carried out with a non-stick applicator.
 5. Themethod of claim 1 wherein the microwave radiation frequency is selectedto be generally between approximately 0.9 GHz to approximately 200 GHz.6. The method of claim 1 wherein the preferable microwave radiationwavelength is selected to be between approximately four opticalthicknesses of said first glass sheet for the selected wavelength toapproximately the sum of the thickness of skin layers in said film andsaid first glass sheet.
 7. The method of claim 1 wherein heating of saidfilm is provided by moving a heat source over the film.
 8. The method ofclaim 1 wherein the vacuum level is selected whereby remaining air inthe laminate does not create visible defects.
 9. The method of claim 1wherein an additional electromagnetic heat source with a wavelength thatis significantly shorter than the applied microwave radiation is used toreheat the film.
 10. The method of claim 9 wherein said additionalelectromagnetic heat source is selected from the group consisting ofinfrared, ultraviolet, laser and X-ray heat sources.
 11. The method ofclaim 1 wherein the step of heating is carried out in a partial vacuum.12. The method of claim 11 wherein the vacuum level is selected wherebyremaining air in the laminate does not create visible defects.
 13. Themethod of claim 1 wherein a reflector is positioned behind said firstglass sheet at a distance equal to 0, 1, 2, 3 . . . multiplied by halfwavelengths of the microwave radiation in a vacuum corresponding to theselected frequency.
 14. The method of claim 13 wherein said reflector isat least in part supporting said first glass sheet.
 15. The method ofclaim 1 wherein at least one combination of steps, selected from thecombinations consisting of the steps of heating and successivelypressing and the steps of reheating and pressing said second glasssheet, is performed simultaneously with an applicator tool comprised ofmaterials transparent to microwave.
 16. The method of claim 15 whereinthe applicator tool material is selected from a group consisting ofTeflon, quartz and oxide ceramic.
 17. The method of claim 1 wherein thevacuum level is selected to be no greater than one kPa.
 18. The methodof claim 1 wherein said areas of heated film are strips and successivelypressing is accomplished with a roller.
 19. The method of claim 1, aftersubjecting said first glass sheet with bonded film to a partial vacuum,stacking multiple of said first glass sheets with bonded film in thepartial vacuum to provide a stack whereby non-coated surfaces of saidfirst glass sheets engage the film bonded to an adjacent first glasssheet with one bonded film left exposed, the steps of positioning andpressing said second glass sheet being carried out on the exposed filmof said stack, and the steps of reheating and cooling each being carriedout on all film layers in said stack simultaneously.
 20. The method ofclaim 19 wherein reheating is carried out on said bonded films of saidfirst sheets prior to stacking.
 21. The method of claim 19 wherein themicrowave radiation frequency is selected such that the temperaturevariation of the stacked films does not exceed a predetermined level.